Keto intramolecular hydrogen bonding

Another feature that will serve to stabilise the enol, with respect to the keto, form is the possibility of strong, intramolecular hydrogen bonding, e.g. in MeCOCH2COMe (31) and MeC0CH2C02Et (23) ... 

Apart from any stabilisation effected with respect to the keto form, such intramolecular hydrogen-bonding will lead to a decrease in the polar character of the enol, and to a more compact, folded-up conformation of the molecule, compared with the more extended conformation of the keto form. This has the rather surprising result that where keto... 

With (32), despite the intramolecular hydrogen-bonding possible in the enol form (326), the equilibrium lies essentially completely over in favour of the keto form (32a), because this can take up an anti-conformation in which the two electronegative oxygen atoms are as far from each other as possible, and in which the carbonyl dipoles are opposed. With (33), the C=0 groups are locked in the syn-con-formation in both keto (33a) and enol (336) forms, and the intramolecular hydrogen-bonding open to (336), but not to (33a), then decides the issue. 

The optically active Schiff bases containing intramolecular hydrogen bonds are of major interest because of their use as ligands for complexes employed as catalysts in enantioselective reactions or model compounds in studies of enzymatic reactions. In the studies of intramolecularly hydrogen bonded Schiff bases, the NMR spectroscopy is widely used and allows detection of the presence of proton transfer equilibrium and determination of the mole fraction of tautomers [21]. Literature gives a few names of tautomers in equilibrium. The OH-tautomer has been also known as OH-, enol- or imine-form, while NH tautomer as NH-, keto-, enamine-, or proton-transferred form. More detail information concerning the application of NMR spectroscopy for investigation of proton transfer equilibrium in Schiff bases is presented in reviews.42-44... 

Electron-donating groups (amino, methylamino, hydroxy, methoxy) in the 2-position, on the other hand, are extremely undesirable because, unlike similar substituents in the 1,4-positions, they are unable to form intramolecular hydrogen bonds with the keto groups of anthraquinone and hence are highly susceptible to photo-oxidation [167]. 

Sucrose Ethers. Being next to the anomeric center and intramolecularly hydrogen-bonded, the 2 -OH of sucrose is the most acidic, which means it is deprotonated first under alkaline conditions, and thus preferentially yields to etherification. Benzylation with NaH/benzylbromide in DMF, for example, results in an 11 2 1 mixture of 2 -(9-benzyl-sucrose (Figure 2.8) and its 1-0- and 3 -0-isomers. Because the former is readily accessible, it proved to be a versatile intermediate for the generation of 2 -modified sucroses, for example, the 2 -keto and 2 -deoxy derivatives as well as sucrosamine (2 -amino-2 -deoxy-sucrose), whose application profiles remain to be investigated. 

Diketones usually exist as mixtures of tautomeric keto and enol forms. The enolic form does not show the normal absorption of conjugated ketones. Instead, a broad band appears in the 1640-1580 cm-1 region, many times more intense than normal carbonyl absorption. The intense and displaced absorption results from intramolecular hydrogen bonding, the bonded structure being stabilized by resonance. 

Table 8.6 shows that the equilibrium mixture consists of almost entirely keto form in the case of simple aliphatic and aromatic ketones, whereas significant amounts of enol tautomer are present in /J-diketones and /J-ketoesters. In these latter cases, the enol contains a conjugated tt electron system and an intramolecular hydrogen bond (30). Phenol exists entirely in the enol form, as the alter-... 

The keto-enol equilibrium of the 1,3-diketones has been the subject of intensive studies using various physical techniques and theoretical calculations [78-80], Recently, X-ray crystal analysis of acetylacetone (83) was carried out at 110 K, and it was found that it exists as an equilibrium mixture of the two enol forms 83b and 83c [81]. Room-temperature studies show an acetylacetone molecule with the enolic H-atom centrally positioned, which can be attributed to the dynamically averaged structure 83d. Application of a crystal engineering technique showed that a 1 1 inclusion complex of83 can be formed with l,l/-binaphthyl-2,2/-dicarboxylic acid in which the enol form is stabilized by a notably short intramolecular hydrogen bond [82],... 

Ordinarily we do not write the enol form of acetone or the keto form of phenol, although minuscule amounts do exist at equilibrium. But both forms of acetylacetone are seen in the NMR spectrum because equilibration is slow enough on the NMR scale and the enol form is stabilized by intramolecular hydrogen bonding. The enol form of acetone and the keto form of phenol are not thus stabilized furthermore, the aro-... 

The a-cyano-a-naphthyl pyruvic acid ethyl ester shows a hydroxyl band at 3190 cm. 1 which is attributed to chelation (very strong intramolecular hydrogen bonding) (2). At 1717 cm. 1 it gives the a,/3-unsaturated carbonyl band of the enol form (lit. Ref. 2, 1715-1730 cm. 1). The phenyl ester shows the same bands at 3190 and 1717.5 cm. 1, respectively, whereas the thiophene ester spectrum shows intermolecular hydrogen bonding at 3310 cm.-1. Since the a-keto ester carbonyl band of the keto... 

Table 1 summarizes these parameters characterizing the keto-enol equilibria, where A refers to the difference between the enol and keto forms. The enol forms are significantly more stable, consistent with the inclusion of an intramolecular hydrogen bond in the structures and concurrent resonance stabilization. The low frequency torsional vibration of the keto forms can account for their significantly greater relative entropy. 

Proton transfer occurs across short intramolecular hydrogen bonds in the gas or liquid phase but is rarely observed in crystalline state. Intra- and intermolecular keto-enol proton transfer C-OH- 0=C C=0-H-0-C occurs in the vapor and liquid states in /7-diketones and in /7-ketoesters where the two states are exactly symmetrically equivalent. Any configuration change which destroys this symmetry inhibits the proton transfer. 

For acetone and the majority of cases in which this keto-enol tautomerism is possible, the keto form is far more stable and little if any enol can be detected. However, with j8-diketones and j8-ketoesters, such factors as intramolecular hydrogen bonding and conjugation increase the stability of the enol form and the equilibrium can be shifted significantly to the right. 

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